Each VC-4 becomes an Administrative Unit-4 when Administrative Unit pointers are added the structure. AU pointers specify where in the AU-4 the VC-4 begins and are located in 9 bytes added to the beginning of the fourth row of the VC-4. This is shown below.
The position of the VC-4 is allowed to vary due to the possibility of a slight difference in frequency between the AU-4 and VC-4. This was not a feature of the protocol we decided to handle; we fixed the value of the AU pointers.